The invention relates to a method of preparation for polymers, particularly binders for powder coatings, and powder coatings which are prepared as a solution in a supercritical fluid phase.
Processes for the preparation of polymers using solvents in the supercritical (sc) state, particularly sc CO2, are already known in the literature. In these processes, monomers or oligomers are dissolved in supercritical solvents and then allowed to react. The resulting polymers are usually insoluble in the supercritical solvent and form two-phase systems. High molecular weights may be obtained with this method of preparation. Such processes are already described, e.g., in U.S. Pat. No. 5,328,972, in which, however, only about 20 wt-% of monomers are present in sc CO2, in EP 0 735 051, which describes a process for polymerisation in supercritical solvents in which homopolymers or copolymers of thermoplastic resins are prepared, in EP 0 220 603, in which polymers in powder form based on special unsaturated vinyl compounds are prepared in sc CO2. In these processes which use, in principle, a large proportion of supercritical solvents, particularly CO2, it is known that resulting polymers are no longer dissolved homogeneously in the fluid phase in the required reaction conversions. A disadvantage of reactions in this phase (emulsion polymerisation, dispersion polymerisation) in supercritical media is the use of expensive stabilisers. Moreover, problems may occur in controlling and conducting the reaction.
Moreover, processes are known in which polymers are liquefied by means of a solvent in the supercritical state, e.g., sc CO2, under pressure and at elevated temperature, wherein hardeners and additives may optionally be introduced into the mixture in the resulting viscous phase and the mixture under pressure is then depressurised in order to obtain materials in powder form and powder coatings. Examples of such processes are described in EP 669 858, WO 99/24 493, and EP 0 744 992 in which a process for the preparation of particles and powders is described with depressurisation of a viscous mixture of the substance to be converted to a powder and a supercritical solvent.
These processes operate according to the sequence whereby the polymer or the individual components of a polymer mixture must be melted, impinged on by the supercritical solvent and homogenised and, after the optional addition of further components, the corresponding powder is obtained after a depressurisation step. A known disadvantage is that of fresh process steps of melting and homogenising the resin material with the supercritical solvent. This is associated with exposure to high temperatures which, if reactive components are used, may lead to side reactions and hence to the unusability of the material.
The object of the invention is to provide a process in which, starting from monomers capable of free-radical or ionic polymerisation and which may be mixed homogeneously in sc fluids, a homogeneously dissolved polymer is produced and this is separated from the supercritical phase. A further object is to provide a process in which, after the polymer has been prepared, said polymer is further processed directly without leaving the supercritical phase state, and hardeners, additives and further components are added in the fluid state and the viscous/fluid mixture is then depressurised in order to obtain an uncross-linked powder coating in powder form.
It has become apparent that the object may be achieved by the process according to the invention for the preparation of binders for powder coatings from at least two ethylenically unsaturated monomers capable of free-radical or ionic copolymerisation and conventional starters and regulators in sc fluids, characterised in that
polymerisation is carried out in the homogeneous phase,
after the reaction has ended either the sc fluid content is increased or the phase parameters (pressure, temperature) of the mixture are altered so that at least two phases with supercritical fluid are obtained, a polymer phase (I) saturated with sc fluid and a second phase (II) containing substantially sc fluid and monomer, and
the polymer phase is separated and the polymer is separated as a powder from the sc fluid by depressurisation.
A further embodiment of the invention separates phase II monomers/sc fluid from the two supercritical phases, mixes the remaining homogeneous phase I polymer/sc fluid with further components such as additives, auxiliaries and hardeners at a temperature at which no cross-linking reactions occur, and the resulting fluid mixture is converted to a solid in powder form by depressurising the sc fluid.